By means of a new technique for reversibly interrupting axonal transport, we have examined the transport of three substances confined to adrenergic nerve fibers (tyrosine hydroxylase, dopamine-Beta-hydroxylase and norepinephrine) as well as of a protein that is more widely distributed in nerves (acetylcholinesterase). Our results indicate that each of these substances is distributed, to markedly differeing degrees, among fractions that are stationary or slowly moving, rapidly transported in a distal direction, or rapidly transported in a proximal direction. In the case of acetylcholinesterase, we have shown that much of the stationary enzyme is situated on the external surface of the nerve cell membrane. We propose to explore the origin and fate of this fraction, with particular attention to the role of axonal transport in supplying it. We propose to extend our studies on the kinetics of rapid transport to test our preliminary observations that adrenergic axons can carry up to two times more material than normal without any charge in the velocity of transport. By examining the behavior of axons subjected to small step gradients of temperature we intend to determine the relationship between the amount of material presented to the transport system and the amount of material actually carried. We also plan studies on the mechanism of transport. Our recent observations indicate that transport in mammalian adrenergic nerves fails at about the same temperature at which unmyelinated axons lose microtubular profiles as seen in the electron microscope. A comparison of microtubular profiles and transport in frog nerves, which are more resistant to cooling, may show whether the correlation between these variables in the rabbit is real or fortuitous. BIBLIOGRAPHIC REFERENCES: B. Cosens, D. Thacker, and S. Brimijoin: Temperature-dependence of rapid axonal transport in sympathetic nerves of the rabbit. J. Neurobiol. 7:339-354, 1976. S. Brimijoin and M.J. Wiermaa: Rapid axonal transport of tyrosine hydroxylase in rabbit sciatic nerves. Brain Res. 121:77-96, 1977.